The prosecutor also gave more detail about how the brutal attack unfolded. The attackers arrived in five SUVs and were armed with automatic machine guns, the statement said. They took position in front of the mosque and its entrances.

Survivors of the incident now at the hospital described massive gunfire and loud explosions at the start of the attack. A number of attackers, some of them masked, then entered the mosque, the statement said.

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The attackers had long beards and hair, were wearing military fatigues and were armed with heavy machine guns, according to the statement. At least one of those who entered the mosque was carrying an ISIS flag, it said.

There has not yet been a claim of responsibility from ISIS or its affiliate in Egypt. However, the attack bears the hallmarks of a strike by ISIS, which maintains a foothold in the north of the Sinai Peninsula and inspires local Islamist extremist groups, despite the efforts of Egyptian security forces.

Victims are seen on the floor of al Rawdah mosque following the gun and bomb attack Friday.

President vows 'brute force'

President Abdel Fattah el-Sisi vowed to respond to the massacre -- believed to be the deadliest terrorist attack on the country's soil -- with "brute force."

Egyptian warplanes conducted airstrikes on "terrorist outposts" and vehicles following the attack, the military said Saturday.

"The Egyptian Air Force pursued the terrorist elements, discovered and destroyed a number of vehicles that carried out the brutal terrorist killings, and killed all terrorists inside those vehicles," military spokesman Tamer Rifai said in a statement Saturday.

It also targeted a number of terrorist outposts containing weapons, ammunition and radical elements, Rifai said.

A teenage survivor of the attack is treated Friday at a hospital in Ismailia.

Al Rawdah mosque is known as the birthplace of an important Sufi cleric. Sufism is a mystical branch of Islam that some jihadists consider heretical.

Witnesses told CNN that the attack started when an explosive device went off in a building adjacent to the mosque and that gunmen fired at worshippers as they fled. Gunmen then entered the mosque and fired at those inside, they said.

The attackers had also set up "ambush" locations and opened fire on ambulances as they transported the wounded to al-Arish before the arrival of security services, according to witnesses.

Photos from inside the mosque showed what appeared to be bodies lined up in rows on the carpet.

'No one got out of the mosque'

One survivor, who asked that his name not be used, told CNN on Saturday that he still had shrapnel in an eye that was covered by a bandage. Sitting with tribesmen outside a hospital emergency room, he said he believes he survived because he was covered by the bodies of other victims as the attackers went around the mosque to ensure everyone was dead.

"No one got out of the mosque," said another witness at a nearby hospital. The man, who also declined to be identified out of fear of retaliation, said his father and eight other family members, including a brother and nephew, were killed.

The man said soldiers from a nearby military unit must have heard the gunfire during the attack but did not immediately respond.

"Even the ambulances came under fire." he said.

A man nearby nodded in agreement. Moments later, hospital volunteers alerted a military officer protecting the hospital -- who asked reporters to leave.

Another man told CNN he drove one of the first ambulances to the scene but turned around after shots were fired at the vehicle. Ambulances from al-Arish managed to reach the mosque only after security forces secured the road, he said.

The attackers used automatic weapons, said Diaa Rashwan, the chairman of the state information service. Some victims were also killed by the explosions.

National mourning declared

In a short, televised speech after meeting with security officials, Sisi said the government would hunt down the attackers.

"We will respond to this act with brute force against these terrorists," he said. "This terrorist act will strengthen our resolve, steadfastness and will to stand up to, resist and battle against terrorism."

President Abdel Fattah el-Sisi, center, meets with officials in Cairo after the mosque attack.

Three days of national mourning have been declared. Sisi has also ordered the armed forces to build a memorial to those killed at the mosque, a statement on his official Facebook page said.

The grand imam of Al-Azhar, the premier religious authority in Egypt, condemned the "barbaric attack" on the mosque.

"The shedding of blood, the violation of the sacred houses of God and the terrorizing of worshippers are acts of corruption on the earth," Ahmed El-Tayyeb said.

The targeting of mosques followed an attack on churches, El-Tayyeb said, adding that Egyptians would prevail over terrorism with solidarity and determination.

People gather at the site of the mosque attack on Friday.

Northern Sinai unrest

Friday's attack prompted wide international condemnation, with US President Donald Trump among the leaders expressing condolences and vowing to continue the fight against terrorism.

Relatives of the victims of the mosque attack sit outside a hospital Saturday in the eastern port city of Ismailia.

If it turns out ISIS militants are behind the latest attack, "it reflects their willingness to continue to do large operations that have large civilian casualties," Timothy Kaldas, a non-resident fellow at the Tahrir Institute for Middle East Policy, told CNN.

There would be two plausible motives for an ISIS attack, he said. First, ISIS considers Sufis to be heretics, making the Sufi mosque a legitimate target in the eyes of ISIS, even though all worshippers probably weren't Sufi.

Second, Kaldas said, ISIS may be taking revenge against members of the Sawarkah tribe, to which most of the people in the area belong, because it has been cooperating with the Egyptian government in its campaign against Wilayat Sinai.